The invention relates to a digital transmission system for transmitting speech signals at a low bit rate and to a transmitter for use in such a system. The transmitter comprises two transmitter channels, and a band-pass filter through which the speech signal to be transmitted is supplied to both said transmitter channels. One of said channels has a first transmission device operating at a given low bit rate for transmitting to the receiver end a first digital signal representative of the frequency information content of the speech signal. The other transmitter channel has an envelope detector, a low-pass filter and a second transmission device operating at an appreciably lower bit rate than said first transmission device for transmitting to the receiver end a second digital signal representative of the amplitude information content of the speech signal. The first and second digital signals are supplied at the receiver end to a receiver having two receiving channels. One of which comprises a decoder which in response to the applied second digital signal produces an output signal corresponding to the envelope of the speech signal, while the other receiving channel includes a pulse modulator, to which the first digital signal and also the envelope signal as a modulating signal are supplied, and an integrator connected to the output of the pulse modulator and from which the recovered initial speech signal is derived via a band-pass filter.
In digital transmission systems theamplitude quantisation is known to give rise to certain deviations between the signal voltage reproduced at the receiver end and the initial signal voltage. These deviations, which produce what is generally referred to as the quantisation noise, are considerable, especially at low bit rates, and they become particularly troublesome with a comparatively low signal voltages or low signal levels.
A digital transmission system of the aforementioned type is described in U.S. Pat. No. 3,518,548.
In this known system a more accurate reproduction and a reduction of the influence of the quantisation noise are obtained by the use of a high degree of comparison. For this purpose in this known system the first transmitter channel is provided with a transmission device which is constituted by a non-uniform pulse delta modulator which transmits the compressed speech signal whilst a parameter which corresponds to the mean slope of the speech signal is transmitted via a second channel having a second encoder which operates at a much lower bit rate than the said non-uniform delta modulator. Thus the bit rate of the system can be reduced to about 10 kbits/second whilst retaining a fairly good transmission quality of the speech signal.
It is an object of the present invention to provide a transmission system of the above-mentioned type which is distinguished by simplicity of construction and moreover enables the bit rate to be further reduced to 4.0 to 7 kbits/second while retaining a reasonably good transmission quality.